It is known that the properties of pigments can be improved by adding additives which have a similar composition to these pigments. By means of such additives, the coloristic properties, such as color strength, transparency and gloss, are improved when the pigments are used in printing inks.
In the case of disazo pigments, such pigment/additive mixtures can be prepared directly during synthesis by crosscoupling to a mixture of the coupling components (DE-B 15 44 507, 12 89 931 and 15 44 534).
Crosscoupling to pigment mixtures where a mixture of the neutral coupling component and a small amount of a coupling component containing an acidic group is used as the coupling component is also known (DE-A 20 12 152).
The preparation of mixtures of pigments and constitutionally similar compounds which carry acidic groups by crosscoupling a tetrazotized mixture of a 4,4'-diaminodiphenyl and a compound which contains an acidic group and 2 diazotizable amino groups, such as 4,4'-diaminodiphenyldisulfonic acid or 4,4'-diaminostilbenedisulfonic acid, to acetoacetarylides, 1-arylpyrazolones or naphthol-AS components is also known (DE-C 21 22 521 and DE-A 33 29 846).
In another variant for improving the pigment properties, the pigments are mixed with additives which are not constitutionally similar to the pigments to be improved (DE-A 34 34 379).
According to the literature, these additives improve the performance characteristics of the pigments, such as color strength, gloss, heat stability, dispersibility and flow behavior in binders.
Recently, pigments having a high resin content have preferably been used for printing progressive yellow proofs, the resin content of the said pigments being 40% by weight or higher. These resin-containing pigments are distinguished in particular by high transparency and, in spite of the high resin content, by a very high color strength and excellent gloss and dispersion behavior. These pigments with a high resin content have the disadvantage that, when they are milled in modern dispersing machines, for example in high speed stirred ball mills, the color strength and the transparency decrease. This is a consequence of the crystallization of the pigments as a result of the temperature increase which occurs during milling.